Disk drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data in a form that can be made readily available to a user. In general, a disk drive comprises a magnetic disk that is rotated by a spindle motor. The surface of the disk is divided into a series of data tracks. The data tracks are spaced radially from one another across a band having an inner diameter and an outer diameter.
Each of the data tracks extends generally circumferentially around the disk and can store data in the form of magnetic transitions within the radial extent of the track on the disk surface. An interactive element, such as a magnetic transducer, is used to sense the magnetic transitions to read data, or to transmit an electric signal that causes a magnetic transition on the disk surface, to write data. The magnetic transducer includes a read/write gap that contains the active elements of the transducer at a position suitable for interaction with the magnetic surface of the disk. The radial dimension of the gap fits within the radial extent of the data track containing the transitions so that only transitions of the single track are transduced by the interactive element when the interactive element is properly centered over the respective data track.
The magnetic transducer is mounted by a head structure to a rotary actuator arm and is selectively positioned by the actuator arm over a preselected data track of the disk to either read data from or write data to the preselected data track of the disk, as the disk rotates below the transducer. The actuator arm is, in turn, mounted to a voice coil motor that can be controlled to move the actuator arm across the disk surface.
A servo system is typically used to control the position of the actuator arm to insure that the head is properly centered over the magnetic transitions during either a read or write operation. In a known servo system, servo position information is recorded on the disk surface between written data blocks, and periodically read by the head for use in a closed loop control of the voice coil motor to position the actuator arm. Such a servo arrangement is referred to as an embedded servo system.
In modern disk drive architectures utilizing an embedded servo, each data track is divided into a number of data sectors for storing fixed sized data blocks, one per sector. Associated with the data sectors are a series of servo sectors, generally equally spaced around the circumference of the data track. The servo sectors can be arranged between data sectors or arranged independently of the data sectors such that the servo sectors split data fields of the data sectors.
Each servo sector contains magnetic transitions that are arranged relative to a track centerline such that signals derived from the transitions can be used to determine head position. For example, the servo information can comprise two separate bursts of magnetic transitions, one recorded on one side of the track centerline and the other recorded on the opposite side of the track centerline. Whenever a head is over a servo sector, the head reads each of the servo bursts and the signals resulting from the transduction of the bursts are transmitted to, e.g., a microprocessor within the disk drive for processing.
When the head is properly positioned over a track centerline, the head will straddle the two bursts, and the strength of the combined signals transduced from the burst on one side of the track centerline will equal the strength of the combined signals transduced from the burst on the other side of the track centerline. The microprocessor can be used to subtract one burst value from the other each time a servo sector is read by the head. When the result is zero, the microprocessor will know that the two signals are equal, indicating that the head is properly positioned.
If the result is other than zero, then one signal is stronger than the other, indicating that the head is displaced from the track centerline and overlying one of the bursts more than the other. The magnitude and sign of the subtraction result can be used by the microprocessor to determine the direction and distance the head is displaced from the track centerline, and generate a control signal to move the actuator back towards the centerline.
Each servo sector also contains encoded information to uniquely identify the specific track location of the head. For example, each track can be assigned a unique number, which is encoded using a Gray code and recorded in each servo sector of the track. The Gray code information is used in conjunction with the servo bursts to control movement of the actuator arm when the arm is moving the head in a seek operation from a current track to a destination track containing a data field to be read or written.
The head structure also includes a slider having an air bearing surface that causes the transducer to fly above the data tracks of the disk surface due to fluid currents caused by rotation of the disk. Thus, the transducer does not physically contact the disk surface during normal operation of the disk drive to minimize wear at both the head and disk surface. The amount of distance that the transducer flies above the disk surface is referred to as the "fly height." By maintaining the fly height of the head at an even level regardless of the radial position of the head, it is ensured that the interaction of the head and magnetic charge stored on the media will be consistent across the disk.
It is expected that users of disk drives will place ever greater demands on disk drive manufacturers with regard to the amount of data that can be stored in and rapidly retrieved from disk drive products. Modern software programs include graphics and other data structures that dramatically increase the amount of data that need to be stored. In addition, the rapid growth in the use of servers on computer networks requires large storage capabilities to accommodate the data needs of a large number of users on the network who utilize the servers.
Accordingly, recent disk drive research and development efforts have focused on the need to continually improve, among other things, the magnetic media used in the disks so as to substantially increase the storage capacity of each new disk drive design to levels sufficient to accommodate the ever greater demands for storage capacity placed on disk drive products by users. The trend in media design is to develop magnetic materials capable of storing magnetic transitions at ever greater densities, both radially and circumferentially, to achieve a total data storage capacity that is greater than heretofore available.
As noted above, due to operation of the air bearing surface, the transducer does not physically contact the disk surface during normal read and write operation of the disk drive. However, it is generally an objective to achieve an overall fly height that brings the read/write gap of the transducer as close to the disk surface as possible. The closer the active read/write gap of the transducer is brought to the surface of the disk, the stronger the electric signal generated by the transducer due to a magnetic transition on the disk surface which represents data. It is generally advantageous to develop as strong a data signal as possible, to insure reliable electrical performance of the disk drive.
Continuing advances in disk drive design that permit lower fly heights make it feasible to further increase the density of magnetic transitions since the smaller transitions that result from greater densities, as measured by the radial and circumferential extent of each transition, can be adequately sensed by the low flying head. A consequence of the increasing capacity of disk drive products having compact designs is that data densities on the surface of the disks and the rotational speeds of the disks are approaching levels that are too high relative to the capability of conventional magnetic transducers to rapidly and accurately sense closely spaced, fast moving magnetic transitions in a data read operation, even at low fly heights.
One proposal to meet the data retrieval requirements of modern disk drive designs is to utilize a magnetoresistive transducer (MR transducer) coupled to an electronic read channel that implements signal processing techniques such as partial response, maximum likelihood detection (PRML read channel). These components provide significantly improved performance capabilities and are able to process signals representative of data at rates suitable for operation with modem high capacity, high performance disk drives.
In an MR head, the transducer comprises a magnetoresistive element that is used to sense the magnetic transitions representing data. The magnetoresistive element comprises a material that exhibits a change in electrical resistance as a function of a change in magnetic flux of a magnetic field applied to the element. In a disk drive environment, the MR element is positioned within the transducer gap, above a disk surface. In this position, the electrical resistance of the element changes in time as magnetic transitions recorded on the disk pass beneath the gap, due to rotation of the disk. The changes in the resistance of the MR element caused by magnetic transitions on a disk occur far more quickly than the response of conventional transducers to magnetic transitions. Thus, an MR transducer is able to sense magnetic transitions at higher rotational speeds and data densities.
The MR transducer is coupled to an electronic circuit, e.g. a pre-amplifier, that operates to detect the resistance changes of the MR element, and generate electrical signals that vary in time as a function of the resistance changes. The pre-amplifier output, therefore, comprises an electrical signal that corresponds to the data recorded as magnetic transitions on the disk surface. The output of the pre-amplifier is coupled to a read channel that thereafter processes the preamplifier output signal according to PRML techniques to interpret the data represented by the output signal. PRML techniques can operate with more efficient data recording codes, and are able to process signals at more rapid rates than conventional peak detectors now widely used in disk drives to detect data from signals received from a transducer.
When the disk drive is not operating, the rotation of the storage disk is stopped, and the air bearing surface of the head ceases to cause the transducer to fly. Under such circumstances, the head, including the transducer, comes to rest on the disk surface. Typically, the actuator arm is operated during power down of the disk drive, to position the head over a landing zone provided on the disk surface away from any of the data tracks. In conventional disk drive products, the landing zone is most often placed at the inner diameter of the disk. The actuator arm is latched when positioned over the landing zone.
In a known contact stop operation of a disk drive, the head comes into contact with the disk surface over the landing zone upon the slowdown and cessation of rotation of the storage disk due to power down of the disk drive. The use of a landing zone prevents any damage to data tracks that may occur due to contact between the head and the disk surface.
A contact start operation, at power up of the disk drive, causes the commencement of rotation of the disk while the head is still in contact with the landing zone. A phenomenon known as "stiction" between the head and the landing zone is a potential problem in a contact start operation. Stiction resists separation between the head and disk surface and can be highly detrimental to disk drive operation. Indeed, the stiction between the disk surface and the head can be so significant that the spindle motor cannot generate sufficient torque to separate the head from the disk surface at all, resulting in a disk drive failure.
When operating at extremely low fly heights, the smoothness of disk surfaces becomes an important design issue. Peaks and valleys in the disk surface, even if of minimal dimensions, can interfere with contact free flying operation of a head when the head is flying at a fly height that is in the same relative dimension range as the peaks and valleys encountered on a disk surface. Thus, another goal of modern disk design is to improve the smoothness characteristics of disk surfaces such that peak and valley dimensions are minimal relative to the low head fly height. However, the greater the smoothness of a disk surface, the greater the potential for unacceptable stiction between the head and disk surface.
A proposed solution to the stiction problem resulting from extremely smooth disk surfaces is to texture the surface of the landing zone. A textured surface increases the roughness of the surface to thereby reduce stiction between the head and disk surface. Thus, the data track portions of the disk are formed to an extremely smooth surface to facilitate a low fly height for an MR head, while the landing zone is formed to a rough surface such that stiction is minimized during a contact start operation.
In a conventional disk drive, the landing zone typically includes a track or tracks containing servo information that is used to provide initial head position information. During the contact start operation, when the head has reached a take-off velocity, the head is activated to read the servo information. The head is then positioned by the actuator arm over the landing zone track, using the recorded servo information as a servo lock. The initial controlled position is a starting point reference to control further radial movement of the actuator arm in an initial seek from the landing zone servo track back out to the data track portions of the disk. In this manner, the actuator arm is moved form the landing zone to a position over a preselected one of the data tracks in a controlled operation.
Absent the use of servo control during the time of the initial seek out of the landing zone, the actuator arm would move blindly, without closed loop control over its speed or position. Under such circumstances, the actuator arm may hit a crash stop provided to limit the radial extent of actuator arm movement, before control is attained over actuator arm movement. This can result in mechanical shock to the actuator arm that causes irreparable damage to the head and/or the media. A problem with the use of a textured landing zone in a disk drive implementing a low fly height MR head is that the MR element cannot reliably transduce transitions recorded on a rough surface when the head is flying at a relatively low fly height. This is due to thermal asperity caused when the MR head contacts the media. Thus, a stable initial seek based upon a servo lock on a landing zone servo track cannot be assured, leaving the disk drive vulnerable to crash stop hits.
Moreover, while the rough surface of the landing zone minimizes stiction problems, the flyability of the head over the rough surface is poor, particularly in a low fly height environment, making the time of flight over the landing zone a significant concern to long work life prospects for the disk drive. The conventional contact start approach of staying in the landing zone until a satisfactory take-off velocity has been achieved by the head can result in excessive time in the rough landing zone causing accelerated wear of the head. A similar problem occurs during a contact stop operation when the head is retracted into the landing zone too far in advance of landing on the rough disk surface.
Accordingly, the significant advances contemplated for disk drive designs raise a new set of problems affecting the robustness of disk drive operation. The use of MR heads with textured media enable an important advance in data storage capacity and speed of operation of the disk drive. However, the resulting loss of control of head movement during an initial seek out of the landing zone at power up of the drive and undue wear on the head caused by excessive time within the textured landing zone can lead to premature disk drive failure.